The present invention relates to an apparatus for developing electrostatic latent images carried on a photosensitive surface, and more particularly to a developing apparatus the type of using a rotatable sleeve.
In electrostatic latent image developing apparatuses in photocopiers, electrostatic recording apparatuses and the like, electrostatic latent images are formed on an image-bearing surface made from selenium or zinc oxide photosensitive materials, organo-photoconductive materials, etc., developed with magnetic developers by way of a magnetic brush method, and fixed on papers, or transferred to transfer sheets and then fixed.
Widely used as such a developing apparatus is that of the magnetic brush type comprising a rotatable sleeve made of a non-magnetic material for supporting a developer and a magnetic roll having a plurality of magnetic poles and installed within the sleeve.
As magnetic developers for the magnetic brush method, two-component developers consisting of ferromagnetic carriers and toners are widely used. The magnetic carries are iron particles, ferrite particles, nickel particles, etc. which may be coated with organic polymers. The toners are fine resin particles containing coloring pigments or dyes dispersed therein. The carrier and the toner are selected so that they are charged to opposite polarities by friction when mixed and stirred.
The basic structure of the magnetic brush-type developing apparatus remains substantially unchanged in recent years, but various proposals have been made so far to improve the quality of images.
For instance, Japanese Patent Laid-Open No. 50-116042 (U.S. Pat. No. 3,952,701) discloses an electrostatic developing apparatus comprising a main developing magnetic poles and a plurality of magnetic poles of the same polarity opposing each other for bringing a wide magnetic brush into contact with a photosensitive drum uniformly. Japanese Patent Laid-Open No. 51-43151 (U.S. Pat. No. 4,030,447) discloses a developing apparatus comprising a magnet roll having magnetic poles of identical polarity disposed adjacent to each other at positions opposed to the latent image bearing surface. This developing apparatus utilizes a powder cloud phenomenon like cascade in a repulsive magnetic field. And Japanese Patent Laid-Open No. 54-122131 discloses a developing apparatus comprising a single magnet for providing a relatively weak magnetic field and a relatively strong magnetic field to a developing site therein. These developing apparatuses use two-component developer.
In a magnetic brush developing method using two-component developers, only the toner charged at a predetermined polarity due to the friction with carrier is adhered ti an image-bearing surface so that the transfer of a toner image is easy. However, since it is necessary to control the concentration of toner during the developing operation, a toner concentration controlling means is needed, making the structure of the developing apparatus rather complicated.
To eliminate such a problem, recently one-component magnetic toner consisting mainly of a resin and magnetic powder has come to be used as a developer. In the magnetic brush developing method using a magnetic toner, a clear transferred image cannot be obtained by the use of a conductive magnetic toner (U.S. Pat. No. 3,909,258). Thus, insulated (non-conductive) magnetic toner having high electric resistance is widely used nowadays. Nevertheless, the non-conductive magnetic toner is disadvantageous in that good development cannot be achieved, though it is easily transferred onto a plain paper. To overcome this disadvantage, a proposal was made to repeatedly contact the toner with a sleeve or a doctor blade to charge the toner while rapidly rotating at least the sleeve (U.S. Pat. No. 4,121,931). As this developing method uses a non-chargeable magnetic toner, it is generally difficult to charge the toner fully, making it necessary to elevate the surface potential of the photosensitive material. Accordingly, the photosensitive material inevitably has a shortened service life.
Thus, the use of chargeable magnetic toner containing charge-controlling agents was proposed (Japanase Patent Laid-Open No. 55-48754, etc.). A method using this type of toner can be conducted at a surface potential of a photosensitive drum substantially on the same level as when the two-component developer is used. In a case where the chargeable magnetic toner is used, the developing apparatus may have a simple structure so that a sleeve rotation system is widely employed.
And when the chargeable magnetic toner is used, the toner tends to agglomerate by electric charge on a sleeve. Accordingly, to prevent the agglomeration, the use of a developer consisting of a mixture of the chargeable magnetic toner and the magnetic carrier was also proposed (U.S. Pat. No. 4,640,880), and such a developer has been put into practical use. In the case of using such a magnetic developer, a developing apparatus of the sleeve rotation system is advantageous in structural simplicity.
Further in the case of using the above chargeable magnetic toner, a so-called jumping developing method by which an image is developed without bringing the toner into contact with the non-image area of an image-bearing surface unlike in the magnetic brush method was proposed (U.S. Pat. No. 4,356,245), and it is now practically employed. This developing method also uses a sleeve rotation system.
As described above, in a developing apparatus for developing an electrostatic latent image by means of various magnetic developers, the sleeve rotation system is most advantageous as a means of conveying the developer from the aspect of structural simplicity. In the sleeve rotation system, the height of the brush of the developer on the sleeve is greatly affected by the strength and distribution of a magnetic flux on the sleeve as compared with a case where the magnet is rotated. In other words, with only slight unevenness of the magnetic flux and disturbance of the magnetic flux density distribution, the quality of the resulting image deteriorates greatly. For this reason, the magnetic flux density distribution was conventionally determined for each developing condition to enhance the image quality.
However, it has been found that simple evaluation of a magnetic flux density and its distribution in a radial direction which has generally been made so far does not necessarily make it possible to find the conditions of providing the best image quality, and that therefore, the change of these parameters does not necessarily ensure high-quality image.